Ultrasound imaging has had limited success in cancer diagnosis because of its inability in routinely detecting low contrast lesions. The ability to detect low contrast objects, as well as present fine spatial detail, is directly related to the size of the basic imaging voxel (i.e., 3-D resolution cell). Due to sound velocity inhomogeneities in the body and th lack of large contiguous acoustic windows, conventional imaging systems hav been restricted to relatively small active apertures, resulting in poor resolution. Results from intraoperative ultrasound suggest, however, that imaging undistorted by body wall effects may produce images capable of actually surpassing other diagnostic tests for the detection of cancer. It is the long-term objective of the research proposed here to overcome the technical problems associated with large apertures, permitting the detectio of low contrast objects using conventional transcutaneous imaging. In particular, large active area (>2500 mm2) 2-D- conformal arrays will be explored for ultrasound imaging deep in the body with resolution approachin an acoustic wavelength. Recent progress in phase aberration correction methods, developed primarily for reduction of beam forming artifacts due to propagation through inhomogeneous media raises the possibility of high quality imaging using conformal arrays. It is the purpose of the research plan proposed here to systematically study the key technical problems inherent in the development of such array systems. Consequently, four major areas of research will be addressed. First phase aberration correction algorithms will be extended to handle both 2-D arrays and large time delay excursions across the imaging aperture. Second, methods for obtaining accurate phase aberration measurements and optimal beams in the presence of a large number of "dead" channels will be studied. Because of the discontinuous character of acoustic windows into the body, large area arrays will generate greatly improved clinical image quality only if optima performance is preserved in the presence of a large number of acoustically inactive elements. Third, 2-D array geometries and conformal array construction methods will be examined to aid the development of clinically relevant conformal probes. And fourth, images of phantoms and in vitro tissue specimens will be constructed to test the hypothesis that a high performance large aperture conformal array system will enhance the detection of low contrast lesions.